Semiconductor memory device with memory cells each including a charge accumulation layer and a control gate

ABSTRACT

A semiconductor memory device includes a memory cell unit, word lines, a driver circuit, and first transistors. The word lines are connected to the control gates of 0-th to N-th memory cells. The (N+1) number of first transistors transfer the voltage to the word lines respectively. Above one of the first transistors which transfers the voltage to an i-th (i is a natural number in the range of 0 to N) word line, M (M&lt;N) of the word lines close to the i-th word line pass through a region above the gate electrode by a first level interconnection without passing over the impurity diffused layers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/098,058, filed Dec. 5, 2013, which is a continuation of U.S.application Ser. No. 12/695,623, filed Jan. 28, 2010, which claims thebenefit of priority from prior Japanese Patent Application No.2009-019678, filed Jan. 30, 2009. The entire contents of each of whichare incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a semiconductor memory device. Moreparticularly, this invention relates to a semiconductor memory devicewith memory cells each including a charge accumulation layer and acontrol gate.

2. Description of the Related Art

An electrically erasable and programmable ROM (EEPROM) has been known asa nonvolatile semiconductor memory capable of rewriting dataelectrically. In addition, a NAND flash memory has been known as anEEPROM capable of high capacity and high integration.

In a NAND flash memory, it is necessary to apply a high voltage to aword line to write or erase data. Therefore, the NAND flash memory isprovided with a row decoder including a transfer transistor fortransferring a high voltage to the word line. Such a configuration hasbeen disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2002-63795.

BRIEF SUMMARY OF THE INVENTION

A semiconductor memory device according to an aspect of the presentinvention includes:

a memory cell unit in which an (N+1) number of memory cells (N is anatural number not less than 1), including charge accumulation layersand control gates formed on the charge accumulation layers, areconnected in series;

an (N+1) number of word lines connected to the control gates of 0-th toN-th memory cells connected in series in a one-to-one correspondence;

a driver circuit which supplies a voltage to the memory cells; and

an (N+1) number of first transistors which are formed on (N+1) number ofelement regions provided in a semiconductor substrate, include gateelectrodes formed above the element regions with gate insulating filmsinterposed therebetween, and transfer the voltage to the word linesrespectively, each of the first transistors including two impuritydiffused layers which are formed at the surface of one of the elementregions and one of which is connected to the driver circuit and theother of which is connected to one of the word lines,

wherein the (N+1) number of element regions are electrically separatedfrom one another and the gate electrodes are connected in common, and

above one of the first transistors which transfers the voltage to ani-th (i is a natural number in the range of 0 to N) word line, M (M<N)of the word lines close to the i-th word line pass through a regionabove the gate electrode by a first level interconnection withoutpassing over the impurity diffused layers.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a block diagram of a flash memory according to a firstembodiment of the invention;

FIG. 2 is a plan view of a memory cell array according to the firstembodiment;

FIGS. 3 and 4 are sectional views taken along line 3-3 and line 4-4 ofFIG. 2, respectively;

FIG. 5 is a plan view of a row decoder according to the firstembodiment;

FIG. 6 is a sectional view taken along line 6-6 of FIG. 5;

FIG. 7 is a circuit diagram of a memory cell unit and a row decoderaccording to the first embodiment;

FIG. 8 is a plan view of a row decoder according to the firstembodiment;

FIGS. 9 and 10 are plan views of a row decoder according to a first anda second modification of the first embodiment, respectively;

FIG. 11 is a plan view of a row decoder according to a second embodimentof the invention;

FIG. 12 is a plan view taken along line 12-12 of FIG. 11;

FIG. 13 is an equivalent circuit diagram of a MOS transistor;

FIG. 14 is an equivalent circuit diagram of a MOS transistor accordingto the second embodiment;

FIG. 15 is a plan view of a row decoder according to a third embodimentof the invention; and

FIG. 16 is a plan view taken along line 16-16 of FIG. 15.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

A semiconductor memory device according to a first embodiment of theinvention will be explained. FIG. 1 is a block diagram of a NAND flashmemory according to the first embodiment.

<Overall Configuration of NAND Flash Memory>

As shown in FIG. 1, the NAND flash memory 1 includes a memory cell array10, a sense amplifier 20, a source line driver 30, a row decoder 40, adriver circuit 50, a voltage generator circuit 60, and a control circuit70.

The memory cell array 10 includes a plurality of memory cell units 11.Each of the memory cell units 11 includes an (n+1) number of memory celltransistors MT0 to MTn ((n+1) is a natural number not less than 2) andselect transistors ST1, ST2. When there is no need to distinguishbetween memory cell transistors MT0 to MTn, they will simply be referredto as memory cell transistors MT. The number of memory cell transistorsMT is, for example, 8, 16, 32, 64, 128, or 256, and is nonlimiting. Eachof memory cell transistors MT has a stacked gate structure including acharge accumulation layer (e.g., a floating gate), formed on asemiconductor device with a gate insulating film interposedtherebetween, and a control gate, formed on the charge accumulationlayer with an inter-gate insulating film interposed therebetween.Adjacent memory cell transistors MT share a source and a drain. Thememory cell transistors MT are arranged between select transistors ST1,ST2 in such a manner that their current paths are connected in series.The drain region on one end side (memory cell transistor MT0) of thememory cell transistors MTs connected in series is connected to thesource of select transistor ST1 and the source region on the other endside (memory cell transistor MTn) is connected to the drain of selecttransistor ST2.

The control gates of memory cell transistors MT0 to MTn in the same roware connected to any one of word lines WL0 to WLn in a common connectionmanner. The gates of select transistors ST1 of the memory cells in thesame row are connected to a select gate line SGD in a common connectionmanner. The gates of select transistors ST2 of the memory cells in thesame row are connected to a select gate line SGS in a common connectionmanner. To simplify the explanation, word lines WL0 to WLn willsometimes simply be referred to as word lines WL.

A plurality of memory cell units 11 connected to the same word line WLand select gate lines SGD, SGS form a memory block. Data is erased inmemory blocks simultaneously. In addition, data is writtensimultaneously into a plurality of memory cell transistors MT connectedto the same word line WL. This writing unit is called a page.

Although not shown in FIG. 1, a plurality of memory blocks is arrangedin a direction perpendicular to the word line WL. The drains of selecttransistors ST1 in the same column are connected to any one of bit linesBL to BLm (m is a natural number) in a common connection manner. Bitlines BL0 to BLm will sometimes simply be referred to as bit lines BL.The sources of select transistors ST2 are connected to a source line SLin a common connection manner. Both of the select transistors ST1, ST2are not necessarily needed. Only one of the select transistors ST1, ST2may be used, provided that the memory cell unit 11 can be selected.

Sense amplifier 20, in a read operation, senses data read from a memorycell transistor MT onto a bit line BL and amplifies the data. In a writeoperation, sense amplifier 20 transfers write data to a bit line BL.More specifically, sense amplifier 20 applies a voltage corresponding towrite data to a bit line BL.

Source line driver 30 applies a voltage to the source line SL.

Row decoder 40 includes MOS transistors 41, 42 provided for select gatelines SGD, SGS respectively, MOS transistors 43-0 to 43-n provided forword lines WL0 to WLn respectively, and a block decoder 44.

One end of the current path of MOS transistor 41 is connected to selectgate line SGD. One end of the current path of MOS transistor 42 isconnected to select gate line SGS. The other ends of MOS transistors 41,42 are connected to signal lines SGDD, SGSD, respectively.

One end of each of MOS transistors 43-0 to 43-n is connected to thecorresponding one of word lines WL0 to WLn. The other end of each of MOStransistors 43-0 to 43-n is connected to the corresponding one of signallines CG0 to CGn. That is, MOS transistors 41, 42 function as transfertransistors that transfer the potentials on signal lines SGDD, SGSD toselect gate lines SGD, SGS, respectively. MOS transistors 43-0 to 43-nfunction as transfer transistors that transfer the potentials on signallines CG0 to CGn to word lines WL0 to WLn, respectively. Hereinafter,when there is no need to distinguish between MOS transistors 43-0 to43-n, they will simply be referred to as MOS transistors 43. Inaddition, when there is no need to distinguish between signal lines CG0to CGn, they will simply be referred to as signal lines CG. The gates ofMOS transistors 41 to 43 connected not only to select gate lines SGD,SGS connected to select transistors ST1, ST2 and memory cell transistorsMT but also to word lines WL in the same memory block are connected tothe same signal line TG.

Block decoder 44 externally receives a block address and decodes theblock address. Then, block decoder 44 selects signal line TG to whichMOS transistor 43 corresponding to memory cell unit 11 that includes aselected memory cell transistor to be written into, read from, or erasedfrom is connected, thereby turning on MOS transistors 41 to 43.

Driver circuit 50 supplies a voltage necessary to write, read, or erasedata to signal line SGDD, SGSD, and CG according to the result ofdecoding the page address. Those voltages are generated by the voltagegenerator circuit 60. The voltages applied to signal lines SGDD, SGSD,and CG will be described in detail later.

Control circuit 70 externally receives a command and controls theoperation of the voltage generator circuit 60 according to the command.That is, in a write, read, or erase operation, or the like, controlcircuit 70 instructs voltage generator circuit 60 to generate a suitablevoltage.

Voltage generator circuit 60 includes a charge pump circuit. Accordingto the instruction given by control circuit 70, voltage generatorcircuit 60 generates a voltage necessary to write, read, or erase dataand supplies the generated voltage to driver circuit 50.

<Configuration of Memory Cell Array 10>

Next, the configuration of memory cell array 10 will be explained indetail.

<Planar Configuration>

First, a planar configuration of memory cell array 10 will be explainedwith reference to FIG. 2. FIG. 2 is a plan view of memory cell array 10.

As shown in FIG. 2, in a semiconductor substrate 80, a plurality ofstrips of element regions AA extending in a first direction is alsoprovided in a second direction, perpendicular to the first direction. Anelement isolating region STI is formed between adjacent element regionsAA. The element isolating regions STI electrically separate the elementregions AA from one another. On the semiconductor substrate 80, thestrips of word lines WL and select gate lines SGD, SGS extending in thesecond direction are formed so as to cross a plurality of elementregions AA. Floating gates FG are provided in the regions where wordlines WL cross element regions AA. Although the width of floating gateFG is narrower than that of element region AA in FIG. 2, the width offloating gate FG may be equal to or greater than the width of elementregion AA. Memory cell transistors MT are provided in the regions whereword lines WL cross element regions AA. In the region where select gatelines SGD cross element regions AA, select transistors ST1 are provided.In the region where select gate lines SGS cross element regions AA,select transistors ST2 are provided. In element regions AA betweenadjacent word lines WL, between adjacent select gate lines, and betweena word line and a select gate line adjacent in the first direction,impurity diffused layers serving as the source regions or drain regionsof memory cell transistors MT and select transistors ST1, ST2 areformed.

An impurity diffused layer formed in element region AA between adjacentselect gate lines SGD in the first direction functions as the drainregion of select transistor ST1. On the drain region, a contact plug CP1is formed. Contact plug CP1 is connected to a bit line BL stripextending in the first direction. An impurity diffused layer formed inelement region AA between adjacent select gate lines SGS in the firstdirection functions as the source region of select transistor ST2. Onthe source region, a contact plug CP2 is formed. Contact plug CP2 isconnected to a source line (not shown).

<Cross-Section Configuration>

Next, a cross-section configuration of memory cell unit 11 will beexplained with reference to FIGS. 3 and 4. FIG. 3 is a sectional viewtaken along line 3-3 of FIG. 2. FIG. 4 is a sectional view taken alongline 4-4 of FIG. 2.

As shown in FIGS. 3 and 4, an n-well region 81 is formed at the surfaceof a p-type semiconductor substrate 80 and a p-well region 82 is formedat the surface of the n-well region 81. At the surface of the p-wellregion 82, a plurality of strips of element isolating regions STIextending in the first direction are also formed in the seconddirection. As a result, a strip of element regions AA extending in thefirst direction is formed, being surrounded by element isolating regionsSTI.

On the element isolating region AA, a gate insulating film 83 is formed.On the gate insulating film 83, the gate electrodes of the memory celltransistors MT and select transistors ST1, ST2 are formed. Each of thegate electrodes of the memory cell transistors MT and select transistorsST1, ST2 includes a polysilicon layer 84 formed on the gate insulatingfilm 83, an inter-gate insulating film 85 formed on the polysiliconlayer 84, and a polysilicon layer 86 formed on the inter-gate insulatingfilm 85. The inter-gate insulating film 85 is formed by using of, forexample, a silicon dioxide film, or an ON, an NO, or an ONO film whichhave a stacked structure of a silicon dioxide film and a silicon nitridefilm, or a stacked structure including those, or a stacked structure ofa TiO₂, HfO₂, Al₂O₃, HfAlOx, or HfAlSi film and a silicon dioxide or asilicon nitride film.

In the memory cell transistors MT, the polysilicon layers 84 function asfloating gates (FG). The polysilicon layers 86 adjacent in the seconddirection are connected to each other and function as a control gate(word line WL). In the select transistors ST1, ST2, the polysiliconlayers 84, 86 adjacent in the second direction are connected to eachother. The polysilicon layers 84, 86 function as select gate lines SGS,SGD. Only the polysilicon layers 84 may function as select gate lines.In this case, the polysilicon layers 86 of the select transistors ST1,ST2 are set at a specific potential or in a floating state. At thesurface of the semiconductor substrate 80 between gate electrodes, ann⁺-type impurity diffused layer 87 is formed. The impurity diffusedlayer 87, which is shared by adjacent transistors, functions as a source(S) or a drain (D). The region between a source and a drain adjacent toeach other functions as a channel region serving as an electron movingregion. These gate electrodes, impurity diffused layers 87, and channelregions form MOS transistors which function as memory cell transistorsMT and select transistors ST1, ST2.

On the semiconductor substrate 80, an interlayer insulating film 88 isformed so as to cover the memory cell transistors MT and selecttransistors ST1, ST2. In the interlayer insulating film 88, a contactplug CP2 reaching the impurity diffused layer (source S) 87 of selecttransistor ST2 on the source side is formed. On the interlayerinsulating film 88, a metal wiring layer 89 connected to the contactplug CP2 is formed. The metal wiring layer 89 functions as a source lineSL. Further, in the interlayer insulating film 88, a contact plug CP3reaching the impurity diffused layer (drain D) 87 of select transistorST1 on the drain side is formed. On the interlayer insulating film 88, ametal wiring layer 90 connected to the contact plug CP3 is formed.

On the interlayer insulating film 88, an interlayer insulating film 91is formed so as to cover the metal wiring layers 89, 90. In theinterlayer insulating film 91, a contact plug CP4 reaching the metalwiring layer 90 is formed. On the interlayer insulating film 91, a metalwiring layer 92 connected to a plurality of contact plugs CP4 in commonis formed. The metal wiring layer 92 functions as a bit line BL. Thecontact plugs CP3, CP4, and metal wiring layer 90 correspond to thecontact plugs CP1 of FIG. 2.

<Detailed Configuration of Row Decoder 40>

Next, a detailed configuration of the row decoder 40 will be explained,particularly focusing on MOS transistor 43.

<Planar Configuration>

First, a planar configuration will be explained with reference to FIG.5. FIG. 5 is a plan view of a region where MOS transistors 43 are formedin the row decoder 40.

As shown in FIG. 5, an (n+1) number of strips of element regions AAextending in the first direction are also provided in the seconddirection in the semiconductor substrate 80. Between element regions AA,element isolating regions STI are formed. The element regions AA areelectrically separated by the element isolating regions STI. On each ofthe element regions AA, a MOS transistor 43 is formed.

Specifically, on each of the element regions AA, the gate electrode 100of a MOS transistor 43 is formed so as to cross the individual elementregions AA in the second direction. Further, in each of the elementregions AA, impurity diffused layers serving as one and the other endsof the current path of the MOS transistor 43 are formed. Here, i in FIG.5 is in the range of 0 to n. MOS transistors 43-0 to 43-n connected tothe same memory block are arranged in a line in the second direction.The gate electrodes 100 of these MOS transistors are connected in commonand function as a signal line TG. Accordingly, the gate electrodes 100form strips extending in the second direction. In the example of FIG. 5,MOS transistors 43-0 to 43-n are arranged, starting with the one closestto the memory cell array 10. That is, MOS transistor 43-0 is providedclosest to the memory cell array 10 and MOS transistor 43-n is providedfarthest away from the memory cell array 10.

On one end of the current path of MOS transistor 34, a contact plug CP10is formed. One end of the current path of MOS transistor 34 is connectedvia contact plug CP10 to a metal wiring layer (M0) 101 in a first levellayer. The metal wiring layer (M0: first level interconnection) is ametal wiring layer in the lowest layer of the NAND flash memory 1. Themetal wiring layer 101 is drawn to the boundary between the row decoder40 and memory cell array 10 and connected to a word line WL. When it isnecessary to distinguish between the metal wiring layers 101 connectedto MOS transistors 43-0 to 43-n, they will be referred to as metalwiring layers 101-0 to 101-n. That is, metal wiring layers 101-0 to101-n are connected to word lines WL0 to WLn, respectively. In otherwords, it may be said that metal wiring layers 101-0 to 101-n functionas part of word lines WL0 to WLn.

The other end of the current path of MOS transistor 34 is connected to ametal wiring layer 102 in the first level layer via a contact plug CP11.The metal wiring layer 102 is connected to a metal wiring layer (M1:second level interconnection) 103 in a second level layer higher thanthe metal wiring layer (M0) via a contact plug CP12. The metal wiringlayer 103 is connected to a metal wiring layer (M2: third levelinterconnection) 104 in a third level layer higher than the metal wiringlayer in the second level layer via a contact plug CP13. The metalwiring layer 104, which functions as a signal line CG, has the form of astrip and extends in the first direction and passes through a regionbetween adjacent element regions AA. The gate electrode 100 is connectedto a metal wiring layer 105 in the first level layer with a contact plugCP14 and is connected to a block decoder 44 with the metal wiring layer105.

In the first embodiment, a MOS transistor 43 connected to a word line WLcloser to select gate line SGD is arranged closer to the memory cellarray 10 in the row decoder 40. Accordingly, metal wiring layers101-(i+1) to 101-n connected to MOS transistors 43-(i+1) to 43-n thattransfer voltages to word lines WL(i+1) to WLn pass over MOS transistors43-0 to 43-i that transfer voltages to word line WLi and word lines WL0to WL(i−1). The word lines WL(i+1) to WLn is closer to select gate lineSGS than a certain word line WLi is. The word line WL0 to WL (i−1) iscloser to select gate line SGD than word line WLi is.

In this case, of metal wiring layers 101-(i+1) to 101-n, metal wiringlayers 101-(i+1) to 101-(i+M) connected to an M number of word linesWL(i+1) to WL(i+M) (M is a natural number not less than 1) adjacent toword line WLi pass over the gate electrode 100 on an element region AAwhere MOS transistor 43-i has been formed. Metal wiring layers101-(i+M+1) to 101-n connected to the remaining word lines WL(i+M+1) toWLn pass over a region between contact plug CP11 (or CP10) and gateelectrode 100.

Accordingly, as shown in FIG. 5, metal wiring layers 101 passing overthe gate electrode 100 of each MOS transistor 43 is shifted at theposition corresponding to each MOS transistor 43.

<Sectional Configuration>

Next, a sectional configuration of a MOS transistor 43 in the rowdecoder 40 will be explained with reference to FIG. 6. FIG. 6 is asectional view taken along line 6-6 of FIG. 5.

As shown in FIG. 6, element isolating regions STI are formed in thesurface of the p-type semiconductor substrate 80, thereby forming anelement region AA surrounded by the element isolating regions STI. Atthe surface of the element region AA, an n-well region 110 is formed. Inthe surface region of n-well region 110, a p-well region 111 is formed.In the surface region of p-well region 111, two separate impuritydiffused layers 112 are formed. The impurity diffused layers 112function as the source or drain of MOS transistor 43.

Above p-well region 111 between impurity diffused layers 112, a gateelectrode 100 of MOS transistor 43 is formed with a gate insulating film113 interposed therebetween. Gate electrode 100 is formed of, forexample, a polysilicon layer. The sectional configuration of gateelectrode 100 has the same stacked structure as that of the gateelectrode of each of select transistors ST1, ST2. The gate insulatingfilm 113 is larger than the gate insulating film 83, which enables MOStransistor 43 to withstand a higher voltage than the memory celltransistors MT and select transistors ST1, ST2.

On the semiconductor substrate 80, an interlayer insulating film 114 isformed so as to cover MOS transistor 43 configured as described above.In the interlayer insulating film 114, a contact plug CP10 reaching oneof the impurity diffused layers 112 and a contact plug CP11 reaching theother of the impurity diffused layers 112 are formed. On the interlayerinsulating film 114, metal wiring layers 101-i and 102, respectively incontact with contact plugs CP10 and CP11, are formed.

Further, on the interlayer insulating film 114, metal wiring layers101-(i+1) to 101-n are formed so as to be sandwiched between metalwiring layers 101-i and 102. Of them, metal wiring layers 101-(i+1) to101-(i+M) are arranged in a region directly above gate electrode 100.The remaining metal wiring layers 101-(i+M+1) to 101-n are arranged in aregion directly above the impurity diffused layer 112 in contact withcontact plug CP11.

<Write Operation of NAND Flash Memory>

Next, a write operation of the NAND flash memory 1 configured asdescribed above will be explained. Hereinafter, a case where charges areinjected into charge accumulation layer 84 to raise the thresholdvoltage of the memory cell transistor MT is called a “0” program. Incontrast, a case where no charge is injected into charge accumulationlayer 84 to prevent the threshold voltage from changing (in other words,a case where the injection of charges is suppressed to a degree that theheld data does not transit to another level) is called a “1” program.FIG. 7 is a circuit diagram of memory cell unit 11 and row decoder 40 ina write operation.

When data is written, voltage generator circuit 60 generates a highpositive voltage VPGM and an intermediate voltage VPASS (<VPGM) underthe control of control circuit 70. Voltage VPGM is a high voltage forinjecting electrons into a charge accumulation layer by FN tunneling.Voltage VPASS is a voltage for turning on a memory cell transistor MT,regardless of held data.

Block decoder 44 decodes a block address and applies a “H”-level signalto the signal lines TG of MOS transistors 41 to 43 connected to thememory block including a memory cell transistor MT into which data is tobe written (referred to as a selected cell). As a result, MOStransistors 41 to 43 turn on.

Furthermore, driver circuit 50 decodes a page address, selects signalline CGi, and applies voltage VPGM to signal line CGi. In addition,driver circuit 50 applies voltage VPASS to signal lines CG0 to CG(i−1)closer to select gate line SGD than signal line CGi. Moreover, drivercircuit 50 applies voltage VISO to any one of an M number of signallines CG(i+1) to CG(i+M) adjacent to signal line CGi and voltage VPASSto the rest. Driver circuit 50 further applies voltage VPASS to theremaining signal lines CG(i+M+1) to CGn. Voltage VISO is a voltage forturning off a memory cell transistor MT, regardless of held data.Voltage VISO is, for example, 0 V. Hereinafter, a case where voltageVISO is applied to signal line CG(i+1) will be explained as an example.

Driver circuit 50 further applies voltage VDD and 0 V to signal linesSGDD and SGSD, respectively. Voltage VDD is a voltage for causing selecttransistor ST1 to transfer “0” program data or preventing selecttransistor ST1 from transferring “1” program data. In other words,voltage VDD is a voltage that turns on select transistor ST1 at the timeof the “0” program and turns it off at the time of the “1” program.

As a result, MOS transistors 41 and 42 transfer VDD and 0 V to selectgate lines SGD and SGS, respectively. MOS transistor 43-i transfersvoltage VPGM to word line WLi (selected word line). MOS transistor43-(i+1) transfers voltage VISO to word line WL(i+1) (unselected wordline). Moreover, MOS transistors 43-0 to 43-(i−1), 43-(i+2) to 43-ntransfer voltage VPASS to word lines WL0 to WL(i−1), WL(i+2) to WLn(unselected word lines).

As described above, transferring the voltages to the word lines WLcauses MOS transistors MT0 to MTi, MT(i+2) to MTn to turn on, forming achannel. In contrast, memory cell transistor MT(i+1) goes off, formingno channel. That is, the channels of memory cell transistors MT0 to MTiare conducting and the channels of memory cell transistors MT(i+2) toMTn are conducting. However, MOS transistors MT0 to MTi and memory celltransistors MT(i+2) to MTn are separated by memory cell transistorMT(i+1). Since 0 V is applied to select gate line SGS, select transistorST2 is off. In contrast, select transistor ST1 turns on or off,depending on program data.

When the “0” program is executed, sense amplifier 20 applies a writevoltage (e.g., 0 V) to a bit line BL. Accordingly, select transistor ST1turns on, transferring 0 V applied to the bit line to the channels ofmemory cell transistors MT0 to MTi. Then, in the memory cell transistorMTi connected to the selected word line WLi, the potential differencebetween the gate and channel is almost VPGM, with the result thatcharges are injected into charge accumulation layer 84. As a result, thethreshold voltage of memory cell transistor MTi rises, causing the “0”program to be executed.

When the “1” program is executed, sense amplifier 20 applies a writeinhibit voltage (e.g., VDD) to a bit line, turning off select transistorST1. Accordingly, the channels of memory cell transistors MT0 to MTi inthe memory cell unit 11 go into an electrically floating state. Then,the potentials at the channels of memory cell transistors MT0 to MTirise as a result of coupling with the gate potentials (VPGM, VPASS).Therefore, in memory cell transistor MTi connected to the selected wordline WLi, the potential difference between the gate and channel isinsufficient, preventing charges from being injected into chargeaccumulation layer 84 (to a degree that the held data transits). As aresult, the threshold voltage of memory cell transistor MTi remainsunchanged, causing the “1” program to be executed.

<Effect>

As described above, the semiconductor memory device of the firstembodiment can suppress a decrease in the voltage transfer capability ofMOS transistor 43 and improve the operation reliability of the NANDflash memory 1. This effect will be explained below.

(1) Local Self-Boost

In the field of NAND flash memories, a method of writing data by aself-boost method has been known. The self-boost method is the techniquefor turning off select transistors ST1, ST2 of memory cell unit 11including the MOS transistors MT that run the “1” program, therebybringing the channels of the memory cell transistors MT included in thememory cell unit 11 into an electrically floating state, which raisesthe potentials at the channels by coupling with the word lines WL. As aresult, in memory cell transistor MTi connected to the selected wordline WLi, the potential difference between the gate and channeldecreases, preventing charges from being injected into the chargeaccumulation layer, which causes the “1” program to be executed.

In the self-boost method, it is important to boost the channel potentialefficiently. The reason for this is that, if the boost is insufficient,there is a possibility that the “0” program will be erroneously executedto the MOS transistors MT to which the “1” program is supposed to beexecuted. If self-boost is performed using data-written MOS transistorsMT, the boost efficiency might decrease, depending on the data held inthe MOS transistors MT.

As explained in FIG. 7, voltage VISO is applied to at least one of theunselected word lines (e.g., word line WL(i+1)). This unselected wordline is closer to source line SL than the selected word line WLi is. Thevoltage VISO causes memory cell transistor MT(i+1) to go off. Thisprevents programmed memory cell transistors MT(i+2) to MTn closer to thesource than memory cell transistor MT(i+1) from contributing toself-boost. Accordingly, the boost efficiency of the channels of memorycell transistors MT0 to MTi can be increased. This method is known aslocal self-boost.

(2) Miniaturization

In recent years, NAND flash memories have been miniaturized more andmore and the size of one memory block has been reduced further. As aresult, the size of a memory block in the direction of the bit line (orthe length in the first direction) is almost equal to the size, in thegate length direction (or the length in the first direction), of anelement region AA in which a MOS transistor 43 is formed. Alternatively,the size of a memory block (or the length in the first direction) ismade less than twice the length, in the first direction, of the elementregion AA.

(3) Problem

When the local self-boost method is used in a NAND flash memoryminiaturized as described above, a problem arises: the transfercapability of MOS transistor 43 decreases.

Specifically, although not shown in FIG. 5, a plurality of MOStransistors 43 are also arranged in the first direction. The memoryblock size becomes almost equal to the size of MOS transistor 43, withthe result that the distance between adjacent MOS transistors 43 in thefirst direction becomes smaller.

It therefore becomes difficult to place the metal wiring layer 101connecting MOS transistor 43 located far away from memory cell array 10to a word line WL in a space between MOS transistors 43 in the firstdirection. Therefore, it is necessary to cause the metal wiring layer101 to pass over MOS transistor 43 located near memory cell array 10.

In a case where the local self-boost method is used, when data iswritten, any one of the metal wiring layers 101 transfers voltage VISO.Voltage VISO is a low voltage, such as 0 V. When the metal wiring layer101 transferring such a voltage passes over the impurity diffused layer112 of MOS transistor 43, the impurity diffused layer 112 might bedepleted. If the impurity diffused layer 112 has been depleted, itsresistance value increases. As a result, the voltage transfer capabilityof MOS transistor 43 decreases.

This problem is particularly serious in MOS transistor 43 that transfersvoltage VPGM. If voltage VPGM is not transferred sufficiently to theword lines WL, the word lines are written erroneously (or the “0”program cannot be written).

(4) First Embodiment

In the configuration of the first embodiment, when attention is focusedon a certain word line WLi, metal wiring layers 101-(i+1) to 101-(i+M)in the first level layer connected to an M number of word lines WL(i+1)to WL(i+M) close to word line WLi on the source side (or SGS side) arearranged as follows. Metal wiring layers 101-(i+1) to 101-(i+M) arearranged so as to pass through the region above the gate electrode 100without passing over the impurity diffused layer 112, when passing overMOS transistor 43-i that transfers a voltage to word line WLi.

Accordingly, when voltage VPGM is applied to word line WLi, metal wiringlayer 101, which transfers voltage VISO, passes over gate electrode 100without passing over impurity diffused layer 112, above MOS transistor43-i. Therefore, metal wiring layer 101 transferring voltage VISO isprevented from adversely affecting impurity diffused layer 112 of MOStransistor 43-i. That is, metal wiring layer 101 can prevent impuritydiffused layer 112 from being depleted and the voltage transfercapability of MOS transistor 43-i from decreasing.

This will be explained with reference to FIG. 8. FIG. 8 is a plan viewof MOS transistors 43-i to 43-(i+M).

As shown in FIG. 8, MOS transistor 43-i transfers voltage VPGM to wordline WLi. In this case, the word line WL to which voltage VISO isapplied is any one of word lines WL(i+1) to WL(i+M). Any one of metalwiring layer 101-(i+1) to 101-(i+M) transfers voltage VISO. FIG. 8 showsa case where voltage VISO is applied to word line WL(i+1). In the layoutof the first embodiment, metal wiring layers 101-(i+1) to 101-(i+M) passover MOS transistor 43-i.

In the first embodiment, when metal wiring layers 101-(i+1) to 101-(i+M)pass over MOS transistor 43-i, they pass over gate electrode 100 withoutpassing over impurity diffused layer 112. Accordingly, even when any oneof metal wiring layer 101-(i+1) to 101-(i+M) transfers voltage VISO, theimpurity diffused layer 112 of MOS transistor 43-i transferring voltageVPGM can be prevented from being depleted.

First Modification of First Embodiment

In the NAND flash memory, VISO can be applied to not only an M number ofunselected word lines closer to the source (or SGS side) than theselected word line WL but also an M number of unselected word lines onthe drain side (or SGD side).

In the layout of FIG. 5, metal wiring layers 101-0 to 101-(i−1)connected to word lines WL0 to WL(i−1) closer to the drain than theselected word line WLi do not pass over MOS transistor 43-i.Accordingly, there is no need to take these wiring lines into account.

However, when metal wiring layers 101-0 to 101-(i−1) pass over MOStransistor 43-i, the wiring layers can be laid out as in the firstembodiment. This example is shown in FIG. 9. FIG. 9 is a plan view ofMOS transistors 43-i to 43-(i−M) according to a first modification ofthe first embodiment, showing a case where MOS transistor 43-i transfersvoltage VPGM and MOS transistor 43-(i−M) transfers voltage VISO. In FIG.9, the shaded regions are metal wiring lines that transfer voltage VPGMor voltage VISO.

As shown in FIG. 9, MOS transistors 43-0 to 43-n are arranged, startingfrom the one farthest away from memory cell array 10. That is, MOStransistor 43-n is located closest to memory cell array 10 and MOStransistor 43-0 is located farthest away from memory cell array 10.Accordingly, in this layout, metal wiring layers 101-0 to 101-(i−1) passover MOS transistor 43-i.

Therefore, in this case, metal wiring layers 101-(i−M) to 101-(i−1) inthe first level layer connected to an M number of word lines WL(i−M) toWL(i−1) close to word line WLi on the drain side are arranged asfollows. When metal wiring layers 101-(i−M) to 101-(i−1) are arranged soas to pass through a region above gate electrode 100 without passingover impurity diffused layer 112, they pass over MOS transistor 43-i.With this arrangement, even when any one of metal wiring layers101-(i−M) to 101-(i−1) transfers voltage VISO, they do not pass overimpurity diffused layer 112 of MOS transistor 43-i transferring voltageVPGM, which prevents impurity diffused layer 112 from being depleted.

Furthermore, in the layout of FIG. 9, metal wiring layers 101-(i+1) to101-n connected to word lines WL(i+1) to WLn closer to the source thanthe selected word line WLi do not pass over MOS transistor 43-i.Accordingly, there is no need to take these wiring lines into account.

Second Modification of First Embodiment

In FIGS. 5 and 9, MOS transistors 43-0 to 43-n have been arrangedsequentially in the second direction in row decoder 40. However, MOStransistors 43-0 to 43-n are not necessarily arranged sequentially.

A layout in such a case will be explained with reference to FIG. 10.FIG. 10 is a plan view of MOS transistors 43-(i−M) to 43-(i+M) accordingto a second modification of the first embodiment, showing a case whereMOS transistor 43-i transfers voltage VPGM and MOS transistor 43-(i+M)transfers voltage VISO. In FIG. 10, the shaded regions are metal wiringlines that transfer either voltage VPGM or VISO.

As shown in FIG. 10, MOS transistors 43-0 to 43-(i−1), 43-(i+1) to43-(i+M) are arranged farther away from memory cell array 10 than MOStransistor 43-i. That is, metal wiring layers 101-(i−M) to 101-(i−1),101-(i+1) to 101-(i+M) pass over MOS transistor 43-i.

In such a case, all of metal wiring layers 101-(i−M) to 101-(i−1),101-(i+1) to 101-(i+M) are laid out as in the first embodiment. That is,metal wiring layers 1-1(i−M) to 101-(i−1), 101-(i+1) to 101-(i+M) arearranged so as to pass through a region above gate electrode 100 withoutpassing over impurity diffused layer 112, when they pass over MOStransistor 43-i. Accordingly, even when any one of metal wiring layers101(i−M) to 101-(i−1), 101-(i+1) to 101-(i+M) transfers voltage VISO, itdoes not pass over impurity diffused layer 112 of MOS transistor 43-itransferring voltage VPGM, which prevents impurity diffused layer 112from being depleted.

In other words, in the configuration where voltage VISO can be appliedto an M number of unselected word lines WL adjacent to the selected wordline WLi, even if J lines (j is a natural number) of the M number ofunselected word lines WL are located closer to the source line than theselected word line WLi and the remaining K lines (K=M−J) are located onthe bit line side, the first embodiment can be applied.

Second Embodiment

Next, a semiconductor memory device according to a second embodiment ofthe invention will be explained. The second embodiment is such that anyone of an M number of metal wiring layers 101 is provided on impuritydiffused layer 112 in the first embodiment to locate the metal wiringlayer in a region where no problem will arise even if the layer isdepleted. Hereinafter, only parts that differ from the first embodimentwill be explained.

In row decoder 40 according to the second embodiment, when attention isfocused on a certain word line WLi, at least one of metal wiring layers101-(i+1) to 101-(i+M) in the first level layer connected to an M numberof word lines WL(i+1) to WL(i+M) closer to the source (or SGS side) thanword line WLi is arranged as follows. At least one of metal wiringlayers 101-(i+1) to 101-(i+M) is arranged so as to pass over impuritydiffused layer 112 and outside contact plug CP10 in the element regionAA when passing over MOS transistor 43-i that transfers a voltage toword line WLi. That is, at least one of metal wiring layers 101-(i+1) to101-(i+M) is arranged so as to be on impurity diffused layer 112 of MOStransistor 43-i and face gate electrode 100, with contact plug CP 10intervening therebetween.

FIG. 11 is a plan view of a region where MOS transistors 43 are formedin row decoder 40 of the second embodiment, showing a case where metalwiring layer 101-(i+1) is laid out as described above. FIG. 12 is asectional view taken along line 12-12 of FIG. 11. As shown in FIG. 11,metal wiring layer 101-(i+1) passes through a region outside contactplug CP10 on impurity diffused layer 112 of MOS transistor 43-i.

<Effect>

The configuration of the second embodiment can also prevent the voltagetransfer capability of MOS transistor 43 from decreasing and produce thesame effect as that of the first embodiment. The effect will beexplained below.

FIG. 13 shows an equivalent circuit of MOS transistor 43 when metalwiring layer 101 transferring voltage VISO is on the MOS transistor 43and passes through a region between contact plug CP10 (or CP11) and gateelectrode 100.

As shown in FIG. 13, the region between contact plug CP10 (or CP11) andgate electrode 100 corresponds to the current path from a signal line CGto a word line WL. Accordingly, when depletion has occurred as a resultof metal wiring layer 101 passing over the region, the resistance valueof the current path increases. That is, a large voltage drop occursbetween node N1 and node N2 in FIG. 13, making it impossible to transfera sufficient voltage to the word line WL.

With the configuration of the second embodiment, however, metal wiringlayer 101 transferring voltage VISO is on the MOS transistor 43 andpasses outside contact plug CP10 (or CP11). FIG. 14 shows an equivalentcircuit of MOS transistor 43 in this case. As shown in FIG. 14, theregion outside contact plug CP10 (or CP11) has no function as a currentpath between the signal line CG to word line WL. Accordingly, even ifthe resistance value of the region increases, the effect on voltagetransfer can be neglected. Therefore, the transfer capability of MOStransistor 43 can be secured sufficiently.

The second embodiment can be applied to FIGS. 9 and 10 explained in thefirst and second modifications of the first embodiment. Specifically, inFIG. 9, any one of metal wiring layers 101-(i−M) to 101-(i−1) may becaused to pass over impurity diffused layer 112 of MOS transistor 43-iand outside contact plug CP10 (or CP11). Moreover, in FIG. 10, eithermetal wiring layers 101-(i−M) to 101-(i−1) or 101-(i+1) to 101-(i+M) maybe caused to pass over impurity diffused layer 112 of MOS transistor43-i and outside contact plug CP10 (or CP11).

Third Embodiment

Next, a semiconductor memory device according to a third embodiment ofthe invention will be explained. The third embodiment is such that ametal wiring layer in the second level layer is used as any one of an Mnumber of metal wiring layers 101 in the first embodiment. Hereinafter,only parts that differ from the first embodiment will be explained.

In row decoder 40 according to the third embodiment, when attention isfocused on a certain word line WLi, at least one of metal wiring layers101-(i+1) to 101-(i+M) connected to an M number of word lines WL(i+1) toWL(i+M) close to word line WLi on the source side (or SGS side) iscaused to pass over MOS transistor 43-i that transfers a voltage to wordline WLi by a metal wiring layer 120 in the second layer. In this case,metal wiring layer 120 passes over impurity diffused layer 112 of MOStransistor 43-i.

FIG. 15 is a plan view of a region where MOS transistors 43 are formedin row decoder 40 of the third embodiment, showing a case where metalwiring layer 101-(i+1) is laid out as described above. FIG. 16 is asectional view taken along line 16-16 of FIG. 15.

As shown in FIG. 15 and FIG. 16, metal wiring layer 101-(i+1) is drawnfrom an element region AA onto an element isolating region STI and thenis connected to metal wiring layer 120 in the second level layer via acontact plug CP15. Metal wiring layer 120, which has a strip formextending in the second direction, passes over MOS transistors 43-i to43-0 and is drawn to the boundary between memory cell array 10 and rowdecoder 40. Then, metal wiring layer 120 is connected to word lineWL(i+1). In this case, metal wiring layer 120 passes over the impuritydiffused layers of MOS transistors 43-i to 43-0.

<Effect>

The configuration of the third embodiment can also prevent the voltagetransfer capability of MOS transistor 43 from decreasing, and producethe same effect as that of the first embodiment. The effect will beexplained below.

In the third embodiment, metal wiring layer 120 that transfers voltageVISO is second level wiring layer. Accordingly, as shown in FIG. 16,metal wiring layer 120 is separated from the surface of impuritydiffused layer 12 by the sum of the film thicknesses of interlayerinsulating film 114 and interlayer insulating film 121. Accordingly,even when metal wiring layer 120 transfers voltage VISO, the change ofimpurity diffused layer 112 into a depletion layer is suppressed.Therefore, the transfer capability of MOS transistor 43 can be securedsufficiently.

The third embodiment can be applied to FIGS. 9 and 10 explained in thefirst and second modifications of the first embodiment. Specifically, inFIG. 9, any one of metal wiring layers 101-(i−M) to 101-(i−1) may bepassed over impurity diffused layer 112 of MOS transistor 43-i by thesecond level metal wiring layer 120. Moreover, in FIG. 10, either metalwiring layers 101-(i−M) to 101-(i−1) or 101-(i+1) to 101-(i+M) may bepassed over impurity diffused layer 112 of MOS transistor 43-i by thesecond level metal wiring layer 120.

As described above, in a semiconductor memory device according to eachof the first to third embodiments, MOS transistors 43 in row decoder 40are arranged as described below. In the region above transfer transistor43-i that transfers a voltage to word line WLi, an M number of wordlines (M<N) close to an i-th word line WLi are arranged in any one ofthe following manners:

(1) The word lines are passed through a region above gate electrode 100by a first level interconnection 101 without being passed throughimpurity diffused layer 112.

(2) The word lines are passed over impurity diffused layer 112 andthrough a region facing to gate electrode 100 with either the firstcontact plug CP10 or second contact plug CP11 intervening therebetween,by a first level interconnection 101.

(3) The word lines are passed over MOS transistor 43-i by a second levelor more interconnection 120 which is located above the first levelinterconnection 101.

With this arrangement, impurity diffused layer 112 of MOS transistor 43that transfers voltage VPGM can be prevented from having a higherresistance, and the operation reliability of the NAND flash memory canbe improved.

In the second embodiment, metal wiring layer 101-(i−1) has passedoutside contact plug CP10 on MOS transistor 43-i (see FIG. 11). However,metal wiring layer 101-(i−1) may pass outside contact plug CP11,depending on layout.

In the third embodiment, metal wiring layer 120 is not necessarilypassed over impurity diffused layer 112 and may be passed over gateelectrode 100. Moreover, metal wiring layer 120 can be third levelinterconnection or more.

Furthermore, in the second and third embodiments, one of an M number ofword lines adjacent to word line WLi has passed outside contact plugCP10 or by second level interconnection 120. However, all of the Mnumber of word lines may pass outside contact plug CP10 or by the secondlevel interconnection 120.

In addition, the first to third embodiments may be combined suitably.Specifically, by combining the second and third embodiments, part of theM number of word lines adjacent to word line WLi may pass outsidecontact plug CP10 and the remaining ones may pass by the second levelinterconnection 120. Moreover, by combining the first and thirdembodiments, part of the M number of word lines adjacent to word lineWLi may pass over gate electrode 100, another part may pass outsidecontact plug CP10, and the remaining ones may pass by the second levelinterconnection 120. Furthermore, not only the M number of word linesadjacent to word line WLi but also all the word lines WL passing overMOS transistor 34-i may be laid out as explained in the first to thirdembodiments.

While in the above embodiments voltage VISO has been 0 V, voltage VISOis not limited to 0 V. For instance, voltage VISO may be a positivevoltage or a negative voltage, provided that voltage VISO can turn offthe memory cell transistors MT. In addition, the memory cell transistorsMT may have a MONOS structure where the charge accumulation layer 84 isformed by an insulating film instead of a conducting film.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A semiconductor memory device, comprising: amemory cell unit including a first memory cell, a second memory cell, athird memory cell, and a fourth memory cell that are connected inseries; a first word line connected to the first memory cell; a secondword line connected to the second memory cell; a third word lineconnected to the third memory cell; a fourth word line connected to thefourth memory cell; a driver circuit configured to apply a voltage tothe first memory cell, the second memory cell, the third memory cell,and the fourth memory cell; and a first transistor including a firstdiffused layer and a second diffused layer, wherein the first diffusedlayer is connected to the first word line, and the second diffused layeris connected to the driver circuit, wherein when data is written intothe first memory cell, a first voltage is applied to the first wordline, a second voltage is applied to the second word line and the thirdword line, and a third voltage is applied to the fourth word line,wherein the first voltage is larger than both the second voltage and thethird voltage, and the third voltage is larger than the second voltage,wherein the second memory cell and the third memory cell are locatedbetween the first memory cell and the fourth memory cell, and whereinthe second word line and the third word line are located above a gateelectrode of the first transistor.
 2. The device according to claim 1,further comprising: a second transistor connected between the secondword line and the driver circuit; a third transistor connected betweenthe third word line and the driver circuit; and a fourth transistorconnected between the fourth word line and the driver circuit, whereineach of the first word line, the second word line, the third word line,and the fourth word line is arranged in a first direction, wherein eachof the first transistor, the second transistor, the third transistor,and the fourth transistor has a gate length in a second directionperpendicular to the first direction, and wherein the first transistor,the second transistor, the third transistor, and the fourth transistorare arranged in the first direction.
 3. The device according to claim 2,wherein the first memory cell, the second memory cell, the third memorycell, and the fourth memory cell are connected between a first selectiontransistor and a second selection transistor, wherein the memory cellunit is connected to a bit line via the first selection transistor, andis connected to a source line via the second selection transistor, andwherein a distance between the first transistor and the memory cell unitis smaller than a distance between the second transistor and the memorycell unit.
 4. The device according to claim 3, wherein the second memorycell, the third memory cell, and the fourth memory cell are locatedbetween the first memory cell and the second selection transistor. 5.The device according to claim 1, wherein the second voltage turns offthe second memory cell and the third memory cell regardless of helddata.
 6. The device according to claim 1, wherein the second voltage isa positive voltage.
 7. The device according to claim 5, wherein thesecond voltage is a positive voltage.
 8. The device according to claim1, further comprising a contact plug formed on the second diffusedlayer, wherein the first transistor is connected to the driver circuitvia the contact plug, and wherein the fourth word line is located in aregion above the second diffused layer and between a gate electrode ofthe first transistor and the contact plug.
 9. A method of controllingoperation of a semiconductor memory device, the semiconductor memorydevice comprising: a memory cell unit including a first memory cell, asecond memory cell, a third memory cell, and a fourth memory cell thatare connected in series, a first word line connected to the first memorycell, a second word line connected to the second memory cell, a thirdword line connected to the third memory cell, and a fourth word lineconnected to the fourth memory cell, a driver circuit configured toapply a voltage to the first memory cell, the second memory cell, thethird memory cell, and the fourth memory cell, and a first transistorincluding a first diffused layer and a second diffused layer, whereinthe first diffused layer is connected to the first word line, and thesecond diffused layer is connected to the driver circuit; and the methodcomprising: controlling the driver circuit, by a control circuit, tosupply at least one of a first voltage, a second voltage, and a thirdvoltage to at least one memory cell of the memory cell unit, andapplying, by the driver circuit to write data into the first memorycell, the first voltage to the first word line, the second voltage tothe second word line and the third word line, and the third voltage tothe fourth word line, wherein the first voltage is larger than both thesecond voltage and the third voltage, and the third voltage is largerthan the second voltage, wherein the second memory cell and the thirdmemory cell are located between the first memory cell and the fourthmemory cell, and wherein the second word line and the third word lineare located above a gate electrode of the first transistor.
 10. Themethod according to claim 9, the semiconductor memory device furthercomprising: a second transistor connected between the second word lineand the driver circuit; a third transistor connected between the thirdword line and the driver circuit; and a fourth transistor connectedbetween the fourth word line and the driver circuit, wherein each of thefirst word line, the second word line, the third word line, and thefourth word line is arranged in a first direction, wherein each of thefirst transistor, the second transistor, the third transistor, and thefourth transistor has a gate length in a second direction perpendicularto the first direction, and wherein the first transistor, the secondtransistor, the third transistor, and the fourth transistor are arrangedin the first direction.
 11. The method according to claim 10, whereinthe first memory cell, the second memory cell, the third memory cell,and the fourth memory cell are connected between a first selectiontransistor and a second selection transistor, wherein the memory cellunit is connected to a bit line via the first selection transistor, andis connected to a source line via the second selection transistor, andwherein a distance between the first transistor and the memory cell unitis smaller than a distance between the second transistor and the memorycell unit.
 12. The method according to claim 11, wherein the secondmemory cell, the third memory cell, and the fourth memory cell arelocated between the first memory cell and the second selectiontransistor.
 13. The method according to claim 9, wherein turning off thesecond memory cell and the third memory cell, regardless of held data,is performed by applying the second voltage.
 14. The method according toclaim 13, wherein the second voltage is a positive voltage.